The past few years have added a number of complexities to eukaryotic protein synthesis, a system already quite complex to begin with. The new mechanistic findings are the rare, alternate forms of initiation termed internal initiation and re-initiation. Both of these initiation schemes have been demonstrated in in vivo and in vitro experiments. Secondly, a number of new activities have been reported which are associated with either normal initiation or one of the two alternate schemes. Third, physical and biochemical experiments have indicated that a more complex series of interactions might occur between translation factors and other factors, ribosomes, or mRNA which suggest a more sophisticated flow scheme for the formation of 80S initiation complexes. This has led us to propose the following specific aims for the next funded period: 1. purification of a new activity which appears to enhance utilization of mRNA 2. characterization of the influence of p67 on eIF-2 function 3. quantitate numerous factor interactions with an emphasis on the mRNA specific initiation factors 4. determine components necessary for scanning 5. determine components necessary for re-initiation The results of these studies should provide us with a clearer picture of the pathway of 805 initiation complex formation and indicate whether the pathway is uniquely ordered or whether some steps may occur in a random order. This basic information may then be used to better understand the various mechanisms that viruses have adopted to cause a shut-off of host protein synthesis while allowing for the efficient synthesis of viral proteins.